The present invention relates to an improvement in a hydraulic reaction force apparatus for providing an optimal steering force corresponding to detected vehicle velocity and steering angle in a power steering system.
Power steering systems have been widespread and installed in various types of vehicles including small cars in recent years. In such a conventional power steering system, an auxiliary power steering force can reduce the driver's steering force to achieve smooth operation and reduce driver's fatigue. There have been provided various types of conventional power steering systems.
Demand has arisen for optimally controlling a conventional power steering system in accordance with various traveling conditions, let alone a steering force matching with steering operation of the driver and a steering angle, so as to provide an optimal auxiliary power steering force. In a vehicle employing a power steering system of this type, a large auxiliary power steering force must be generated for steering during parking (i.e., a stationary swing) or steering during low-velocity traveling to achieve light and smooth steering. However, when such a large auxiliary power steering force is produced during high-velocity traveling, the driver's steering force of the steering wheel is excessively decreased to cause anxiety of the driver. In practice, such a power steering system is not preferable to optimize driving feelings. Therefore, while the vehicle is traveling at a high velocity, the auxiliary power steering force is decreased to load the steering wheel, i.e., to increase a driver's steering force, thereby guaranteeing stability of straight traveling at a high speed.
In order to decrease the power steering force which occurs in vehicles traveling at a high velocity to load the steering wheel, (i.e. to increase a drivers steering force), a conventional hydraulic reaction force apparatus is adapted to apply an optimal steering reaction force to the steering wheel and control stiffness (i.e., steering reaction force) of the steering wheel at high and low vehicle velocities, as described in Japanese Patent Disclosure No. 59-114159. In the hydraulic reaction force apparatus, a pressure control valve is disposed midway along a branch path which is branched from a hydraulic path for connecting a pump and a selector valve and which is connected to a hydraulic reaction force chamber of the selector valve. The opening of the pressure control valve is controlled by a pilot pressure, so that one end of a pilot pressure control valve is connected to the branch path through an orifice and the other end thereof is connected to a low-pressure path connected to the tank. Such a conventional structure is effective to some extent in reaction force hydraulic control for the hydraulic reaction force chamber to perform steering force control in response to the vehicle traveling conditions.
However, in the conventional hydraulic reaction force apparatus as described above, a pressure of the oil supplied to the hydraulic reaction force chamber must be controlled, so that a pressure control valve and a pilot pressure control valve therefor are required. The number of constituting members is increased, and piping is cumbersome. As a result, the overall structure is complicated and the apparatus as a whole is bulky. Furthermore, the response characteristics of the conventional hydraulic reaction force apparatus are poor in the above-mentioned steering force control. In particular, in order to perform fine adjustment and improve the response characteristics, demand has arisen for a simple implementation which provides good response characteristics and guarantees optimal steering force control.
In the hydraulic reaction force apparatus, therefore, the steering reaction force must be optimally controlled in accordance with the vehicle traveling conditions such as a vehicle velocity and a steering angle so as to provide optimal driving feelings.